Animations: improve movie compatibility when using ffmpeg
Animations: improve movie compatibility when using ffmpeg

Add the -pix_fmt yuv420p option when using ffmpeg to save
animations with the mov or mp4 commands. This improves
compatibility of the movie files since many players do not
support the ffmpeg defaults.

To do this, add a third positional argument to `buildanimation`
to specify if we want an animated GIF file.  The default
is `true`.

See the following for more information:
- https://trac.ffmpeg.org/wiki/Encode/H.264 ('Compatibility')
- https://apple.stackexchange.com/questions/166553/why-wont-video-from-ffmpeg-show-in-quicktime-imovie-or-quick-preview

tbreloff/Plots.jl